Specifies whether data received from the data port is clocked using the DSU internal clock or by an external clock provided by the DTE connected to the port. When an external clock is used, it must be synchronized to the same clock source as the DSU. The DSU Receive Clock is equivalent to the Data Port Transmit Clock. internal(1) The clock is provided internally by the DSU/CSU on the TXC interchange circuit DB (CCITT 114). external(2) The clock is provided externally by the DTE on the XTXC interchange circuit DA (CCITT 113).